Vapor compression systems (VCSs) have helped shape modern civilization and population demographics. Increased access to electricity in the later part of the 19th century and better performance and operational characteristics of VCSs triggered the gradual replacement of absorption refrigeration systems, ARSs the cooling system of that time, with VCSs. Despite their great benefits, VCSs consume significant electrical energy and use refrigerants that are not environment friendly. As much as 72% of the U.S. electricity consumption is to buildings where a large proportion is used for space cooling/heating, dehumidification, and hot water heating. The use of these amenities is rapidly rising in developing countries. Refrigerant emission from VCSs in developing countries is also increasing. The development of more energy efficient technologies can greatly benefit the world's energy and environmental sustainability while sustaining or improving the quality of life. ARSs can use low-grade thermal energy, such as waste heat and solar-thermal energy, for cooling and heating; which can allow development of high performance, inexpensive, and robust systems.
In attempts to build compact and inexpensive systems, alternative heat exchanger configurations and system architectures have been examined. Depending on the choice of working fluid pairs, absorption systems are classified as ammonia-water or lithium bromide-water systems. LiBr-water systems use water as the refrigerant and LiBr solution as the absorbent. The size of heat exchangers required for absorption and desorption of water must account for the limited water mass transfer coefficient of LiBr solution. By enhancing the absorption and desorption transport processes, compact heat exchanger architectures can be developed for economical small-scale heating and cooling systems.
The absorbers and desorbers commonly employed in LiBr-water ARSs involve a falling film over horizontal or vertical tubes or are in a pool boiling configuration where water is boiled from a pool of LiBr solution. Falling-film desorbers are more suitable with low temperature heat sources. In a falling film desorber, LiBr solution is sprayed over a tube bundle to generate thin solution films over the tubes to facilitate a higher rate of water desorption while the heating medium flows within the tubes. At low surface temperatures, water directly diffuses out of the solution film until the solution temperature is insufficient to sustain a sufficient water vapor pressure above the solution.
To enhance the efficiency of these absorbers and desorbers, thin film systems have been investigated recently. Although the earlier work indicated that the absorption rates were only about half of that achievable with the conventional absorbers, the development of ultra-thin film absorbers, Moghaddam et al., WO2013/063210; Isfahani et al., International Journal of Heat and Mass Transfer, 2013, 63, 82-90; and Isfahani et al., International Journal of Multiphase Flow, 2014, 58, 27-38, identifies an absorber structure using a nanofiber membrane where the absorption rate is improved to beyond that possible from conventional absorbers. In like manner, an ultra-thin film absorber can function as an ultra-thin film desorber when the cooling source employed in the absorber is replaced with a heating source to drive desorption. Hence, a system employing ultra-thin film absorbers and desorbers for systems that can provide improved water heating, dehumidification and/or evaporative cooling is desirable.